Hello all wondering if someone can help with an issue I am h kotlinlang #getting-started

Hello all, wondering if someone can help with an issue I am having combining multiple flows. I am trying to merge multiple flows into one flow, the elements in the flow are of a super class. When I collect the flow I am only ever seeing elements from one of the flows. Code in comments.

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open class Person()
class Student(): Person()
class Teacher(): Person()

DB queries:

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@Query("SELECT * from students WHERE important = 1")
fun observeStudents(): Flow<List<Person>>

@Query("SELECT * from teachers WHERE important = 1")
fun observeTeachers(): Flow<List<Person>>

Merge the flows:

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fun observeImportantPeople(): Flow<List<Person>> {
   val students = studentDao.observeStudents()
   val teachers = teacherDao.observeTeachers()
   return merge(students, teachers)
}

With 2 students and one teacher, I will see either 2 students in the flow list or 1 teacher in the flow list. Any ides?

Sam

06/26/2023, 1:45 PM

If you want one list containing the latest students and the latest teachers, you can use `combine`: https://kotlinlang.org/api/kotlinx.coroutines/kotlinx-coroutines-core/kotlinx.coroutines.flow/combine.html

Billy Newman

06/26/2023, 1:51 PM

Thanks Sam, unfortunately when using combine I get a compile time error:

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Type mismatch.
Required:
Iterable<Flow<TypeVariable(T)>>
Found:
Flow<List<Person>>

Sam

06/26/2023, 1:51 PM

What does your code look like? I think it should be something like this:

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fun observeImportantPeople(): Flow<List<Person>> {
   val students = studentDao.observeStudents()
   val teachers = teacherDao.observeTeachers()
   return combine(students, teachers) { s, t -> s + t }
}

Billy Newman

06/26/2023, 1:52 PM

Ah, was just using

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combine(students, teachers)

Didn’t realize you have to provide a function specifying how to combine the lists Thank you!

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